<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>计算器</title>
    <script type="text/javascript">

        window.onload = function () {

            var oInput01 = document.getElementById('input01');
            var oInput02 = document.getElementById('input02');
            var oFuhao = document.getElementById('fuhao');
            var oBtn = document.getElementById('btn');

            oBtn.onclick = function(){
                var num01 = oInput01.value;
                var num02 = oInput02.value;
                var fuhao = oFuhao.value;

                //验证是否为空
                if(num01==''||num02==''){
                    alert('输入框不能为空！');
                    return;
                }
                if(isNaN(num01)||isNaN(num02)){
                    alert('请输入数字！');
                    return;
                }

                switch (fuhao){
                        //这个value是字符串！
                    case "0":
                        alert((parseFloat(num01)*100+parseFloat(num02)*100)/100);
                        break;
                    case '1':
                        alert((parseFloat(num01)*100-parseFloat(num02)*100)/100);
                        break;
                    case "2":
                        alert(((parseFloat(num01)*100)*(parseFloat(num02)*100))/10000);
                        break;
                    case '3':
                        alert(((parseFloat(num01)*100)/(parseFloat(num02)*100)));
                }
            }
        }
    </script>
</head>
<body>
<h1>计算器</h1>
<input type="text" name="" id="input01">
<select id="fuhao">
    <option value="0">+</option>
    <option value="1">-</option>
    <option value="2">*</option>
    <option value="3">/</option>
</select>
<input type="text" name="" id="input02">
<input type="button" name="" value="计算" id="btn">
</body>
</html>